This invention relates to Surface Acoustic Wave (SAW) devices and more particularly to a SAW device configured as a resonator.
SAW resonators have potential features of high frequency fundamental mode operation, high Q, small size and rugged and inexpensive construction. Their usage is however, limited due to several difficulties. First, present SAW resonators have limited selectivity due to the direct transmission of input signals to the output. This is because know SAW resonators make use of bidirectional transducers for both the input and output ports.
The second difficulty arises from the fact that known SAW resonators rely upon bidirectional transducers to detect standing waves to produce the output signal. Since a standing wave is the algebraic combination of traveling waves which are moving in opposite directions, and has relatively stationary maxima and minima, it is vital to place the output transducer in a precise manner so as to enable the standing wave's maxima to be sensed. As a practical matter, the precise location of the standing wave maxima is very difficult to control to within a small fraction of a wavelength. Moreover changes in the standing wave due to aging, temperature variations etc. will cause a shift in the resonant frequency of the device. Finally bidirectional transducers, if mislocated, impair signal coupling to the desired mode of oscillation and may couple to other cavity modes which are undesired. In practice, therefore, it is extremely difficult to produce SAW resonators which exhibit satisfactory output characteristics.
It is therefore an object of this invention to provide a SAW resonator with input/output ports which are not position sensitive.
A further object of this invention is to provide a SAW resonator which substantially eliminates the direct feed of signals from input to output port and thereby enables increased selectivity.